Three-Dimensional P and S Wave Velocity Structure in the Focal Region of the 1984 Western Nagano Prefecture Earthquake.

Abstract

The detailed three-dimensional P and S wave velocity structure in the focal region of the 1984 Western Nagano Prefecture Earthquake occurring close to an active volcano, Mt. Ontake, central Japan, is derived from a tomographic inversion of travel time data obtained by the 1986 Joint Seismological Research in Western Nagano Prefecture. The data set includes 7,693 P-wave and 6,070 S-wave arrival times observed at 49 stations from 212 local earthquakes and 2 explosions. The velocities in the shallow portion of upper crust are determined at each grid point with its spacing of 1-2 km and good resolutions are obtained from the Earth's surface to a depth of 4 km. There exist strong lateral heterogeneities especially from the surface to a depth of 1 km. The seismic velocity map obtained in the height of 1 km above sea level well corresponds to the maps of surface geology and Bouguer gravity anomaly. This correspondence, however, is not so clear in the deeper layers. And, the greater the depth, the less heterogeneous the velocity structure. The velocity distribution on the fault seems to correspond to the distribution of dislocation and rupture front estimated from an analysis of strong motion and geodetic data: the low velocity region has the large amount of dislocation and the retarded rupture front.

High resolution 3-D velocity structure in the source region of the 2000 Western Tottori Earthquake in southwestern Honshu, Japan using very dense aftershock observations

“…Consequently the high velocity anomalies show high resistivity. Hirahara et al (1992) investigated 3-D velocity structure in the source region of the 1984 Western Nagano Prefecture Earthquake. They found that the areas with large dislocations corresponded to those with relatively low velocities on the fault plane.…”

“…Hirahara et al (1992) studied the relationship between the slip distribution of the 1984 Western Nagano Prefecture Earthquake (Mj = 6.9) and the 3-D velocity structure in the source region. They found that the area with large dislocations corresponded to low velocity anomalies on the fault plane.…”

Past, current and future of Japanese national program for earthquake prediction research

“…It was intended to understand the swarm activities and the mainshock-aftershock activity which occurred in the same area. This is the first joint observation for the inland earthquake by many institutions with modern instruments including seismic, geodetic, gravity measurements (e.g., Horiuchi et al, 1992;Hirahara et al, 1992;Kasaya et al, 2002). In the later 5-year programs, the joint observations, where many institutions jointed with their instruments and sheared the data, were conducted several times; in the area of Japanese Alps in central Honshu (e.g., Matsubara et al, 2000), Nikko area in northeastern Honshu (Horiuchi et al, 1997), Tohoku area in northeastern Honshu (e.g., Nakajima et al, 2001), Hokkaido (Katsumata et al, 2003), and southwestern Honshu, Japan.…”

A Real-Time Processing System of Seismic Wave Using Personal Computers.

“…Using data obtained by this dense joint network, Horiuchi et al (1992), Yamazaki et al (1992), Hirahara et al (1992) and Ikami et al (1992) determined detailed hypocenter locations, spatial distribution of focal mechanisms, and three-dimensional velocity structure, respectively.…”